They said they failed to register what was paid to put up Tory activists involved in the party's Battlebus operation - following a Channel 4 investigation.

The party said the tour was part of the national campaign organised by Conservative Campaign HQ and, as such, the expenditure did not form part of the spending by individual candidates.

It says it was "some millions below" the spending threshold for the national campaign and would have had no reason not to declare it.

But Channel 4 said its investigation suggested the operation was focused at a local level, so should have been declared as such.

The deadline for any prosecutions is 12 months from the date of any offence, unless police or the director of public prosecutions apply to the courts for an extension.

'Absence of investigations'

The Electoral Commission is investigating whether the Conservatives met their reporting obligations under the Political Parties Elections and Referendums Act for the 2015 general election and by-elections in Newark, Clacton and Rochester and Strood.

It has the power to fine parties over their national campaign spending - but it is up to the police to investigate or sanction local party candidates over local campaign spending under the Representation of the People Act.

The commission said its view was that "in the absence of any current investigation by the police, it would be sensible for the criminal justice agencies to retain the ability to take action, should appropriate evidence come to light as part of the commission's own investigation".

The Electoral Commission said it does not believe it will finish its investigation before the deadline for any potential offences under the Representation of the People Act expires.